Trouble with js/downloader.agent

I don’t the how it happen , but since last few days whenever I opened my site , my AVG would pop up warning about some malicious javascript . I never paid much attention to this , afterall they even detect harmless keygens as trojans too
. So I never took this seriously , hoping that everything will be fine in few days ( height of optimism ) .
But AVG kept on threatening me , and when my readers complained about this too , I got a little worried . I knew there was a javascript , and I know it was something related to a downloader agent . I knew where to start from . I google it straight away ( Bless google . Without you , we would be helpless ) . But mr. google failed as the only information I could gather that it was some phishing attack ( XSS , iframe ) but didn’t knew how to fix this problem .
Starting from scratch , I checked all my files for something “malicious” . And guess what I got . This bit of code was hiding in header.php .
No idea where this thing came from . Lesson learnt : always take your antivirus seriously and a XSS / iframe attack even more seriously .







